Silicone molding compounds have been known for many years and their uses include, among others, the encapsulation of electrical and electronic devices. Because of their excellent thermal stability and dielectric properties, and their usefulness over a wide temperature range, silicone resins are widely used in this application. However, one of the problems with silicone molding compounds is the fact that their crack resistance leaves something to be desired. This is true both with respect to external stresses brought about by mechanical means and internal stresses caused by rapid fluctuations in temperature. When the encapsulant on an electronic device cracks, foreign material can come into contact with the device and cause failure. Consequently, it is important that the crack resistance of silicone molding compounds be improved.
Silicone-epoxy copolymers and mixtures are also well known both for coating applications and for molding applications. The problems involved in coatings are generally different from those involved in molding and encapsulating. A coating material is usually applied in relatively thin section and, hence, is less subject to internal stresses which cause cracking than encapsulating material which is generally employed in thicker section. Furthermore, the coating material is in contact with the surface of a substrate, whereas an encapsulating material envelopes the substrate. For this reason, stresses built up in encapsulants differ in order of magnitude from those encountered by coatings.
It is known from such patents as U.S. Pat. No. 3,170,962 that copolymers of organosiloxanes and epoxies can be used in coating applications in which the weight percent of epoxy varies from 1 to 99 percent of the total combination. It is also known that from U.S. Pat. No. 3,368,893 that electrophotographic coatings can be made by bonding zinc oxide with silicone epoxy combinations varying from 0 to 100 percent epoxy and this patent specifically shows in Table III a combination of 10 percent epoxy and 90 percent silicone used to bond zinc oxide.
However, when one comes to the use of silicone-epoxies in molding applications, such as copolymers shown by U.S. Pat. No. 3,842,141, the combination is used in the amount of 60 to 85 percent by weight epoxy and from 40 to 15 percent by weight of silicone. The same is true in U.S. Pat. No. 3,971,747 which shows blends of silicone and epoxies used in molding applications in which the amount of epoxy ranges from 40 to 85 percent by weight of the total resin combination. Thus, it can be seen that in molding applications, the art does not suggest the use of lower amounts of epoxy within the range specified in this application.
Applicant has discovered that when the epoxy resin is employed in an amount of from 4 to 15 percent by weight based on the weight of the total silicone and epoxy resin, improved crack resistance as shown by improved thermal shock is obtained in a silicone molding compound containing siliceous fillers. Not only is the crack resistance of the molding compound improved, but also there is an improvement in the resistance to salt water and the compositions are less flammable than the silicone-epoxy molding compounds of the above patents. Flammability increases with increasing epoxy resin content.